Phenol (also known as carbolic acid, phenolic acid, or benzenol) is an aromaticity organic compound with the molecular chemical formula .[
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The molecule consists of a phenyl group () bonded to a hydroxy group (). Mildly acidic, it requires careful handling because it can cause .[ It is acutely Toxicity and is considered a health hazard.][
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Phenol was first extracted from coal tar, but today is produced on a large scale (about 7 million tonnes a year) from petroleum-derived feedstocks. It is an important industrial commodity as a precursor to many materials and useful compounds, and is a liquid when manufactured.[ Phenol and its chemical derivatives are essential for production of , epoxy, Explosive such as picric acid, Bakelite, nylon, , such as phenoxy herbicides, and numerous pharmaceutical drugs.]
Properties
Phenol is an organic compound appreciably solubility in water, with about 84.2 g dissolving in 1000 ml (0.895 M). Homogeneous mixtures of phenol and water at phenol to water mass ratios of ~2.6 and higher are possible. The sodium salt of phenol, sodium phenoxide, is far more water-soluble. It is a combustible solid (NFPA rating = 2). When heated, phenol produces flammable vapors that are explosive at concentrations of 3 to 10% in air. Carbon dioxide or dry chemical extinguishers should be used to fight phenol fires.[
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Acidity
Phenol is a weak acid, with a pH range of 5 to 6. In aqueous solution in the pH range ca. 8 - 12 it is in equilibrium with the phenolate anion (also called phenoxide or carbolate):
- of the phenoxide anion]]
Phenol is more acidic than aliphatic alcohols. Its enhanced acidity is attributed to resonance stabilization of phenolate anion. In this way, the negative charge on oxygen is delocalized on to the ortho and para carbon atoms through the pi system.[ Organic Chemistry 2nd Ed. John McMurry ] An alternative explanation involves the sigma framework, postulating that the dominant effect is the inductive effect from the more electronegative sp2 hybridised carbons; the comparatively more powerful inductive withdrawal of electron density that is provided by the sp2 system compared to an sp3 system allows for great stabilization of the oxyanion. In support of the second explanation, the p Ka of the enol of acetone in water is 10.9, making it only slightly less acidic than phenol (p Ka 10.0).[ Thus, the greater number of resonance structures available to phenoxide compared to acetone enolate seems to contribute little to its stabilization. However, the situation changes when solvation effects are excluded.
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Hydrogen bonding
In carbon tetrachloride and in alkane solvents, phenol hydrogen bonds with a wide range of Lewis bases such as pyridine, diethyl ether, and diethyl sulfide. The enthalpies of adduct formation and the IR frequency shifts accompanying adduct formation have been compiled.[Drago, R S. Physical Methods For Chemists, (Saunders College Publishing 1992), ISBN 0-03-075176-4] Phenol is classified as a HSAB theory.[Laurence, C. and Gal, J-F. Lewis Basicity and Affinity Scales, Data and Measurement, (Wiley 2010) pp 50-51 ISBN 978-0-470-74957-9][ The plots shown in this paper used older parameters. Improved E&C parameters are listed in ECW model.]
Tautomerism
Phenol exhibits keto-enol tautomerism with its unstable keto tautomer cyclohexadienone, but the effect is nearly negligible. The equilibrium constant for enolisation is approximately 10−13, which means only one in every ten trillion molecules is in the keto form at any moment.
For substituted phenols, several factors can favor the keto tautomer: (a) additional hydroxy groups (see resorcinol) (b) annulation as in the formation of , and (c) deprotonation to give the phenolate.
Phenoxides are stabilised by aromaticity. Under normal circumstances, phenoxide is more reactive at the oxygen position, but the oxygen position is a "hard" nucleophile whereas the alpha-carbon positions tend to be "soft".
Reactions
Phenol is highly reactive toward electrophilic aromatic substitution. The enhanced nucleophilicity is attributed to donation pi electron density from O into the ring. Many groups can be attached to the ring, via halogenation, acylation, sulfonation, and related processes.
Phenol is so strongly activated that bromination and chlorination lead readily to polysubstitution.
Aqueous solutions of phenol are weakly acidic and turn blue litmus slightly to red. Phenol is neutralized by sodium hydroxide forming sodium phenate or phenolate, but being weaker than carbonic acid, it cannot be neutralized by sodium bicarbonate or sodium carbonate to liberate carbon dioxide.
When a mixture of phenol and benzoyl chloride are shaken in presence of dilute sodium hydroxide solution, phenyl benzoate is formed. This is an example of the Schotten–Baumann reaction:
Phenol is reduced to benzene when it is distilled with zinc dust or when its vapour is passed over granules of zinc at 400 °C:
When phenol is treated with diazomethane in the presence of boron trifluoride (), anisole is obtained as the main product and nitrogen gas as a byproduct.
Phenol and its derivatives react with iron(III) chloride to give intensely colored solutions containing phenoxide complexes.
Production
Because of phenol's commercial importance, many methods have been developed for its production, but the cumene process is the dominant technology.
Cumene process
Accounting for 95% of production (2003) is the cumene process, also called Hock process. It involves the partial redox of cumene (isopropylbenzene) via the Hock rearrangement:
A route analogous to the cumene process begins with cyclohexylbenzene. It is oxidized to a hydroperoxide, akin to the production of cumene hydroperoxide. Via the Hock rearrangement, cyclohexylbenzene hydroperoxide cleaves to give phenol and cyclohexanone. Cyclohexanone is an important precursor to some .
Oxidation of benzene, toluene, cyclohexylbenzene
The direct oxidation of benzene () to phenol is possible, but it has not been commercialized:
Nitrous oxide is a potentially "green" oxidant that is a more potent oxidant than O2. Routes for the generation of nitrous oxide however remain uncompetitive.
An electrosynthesis employing alternating current gives phenol from benzene.
The oxidation of toluene, as developed by Dow Chemical, involves copper-catalyzed reaction of molten sodium benzoate with air:
The reaction is proposed to proceed via formation of benzyoylsalicylate.
Autoxidation of cyclohexylbenzene gives the hydroperoxide. Decomposition of this hydroperoxide affords cyclohexanone and phenol.
Older methods
Early methods relied on extraction of phenol from coal derivatives or the hydrolysis of benzene derivatives.
Hydrolysis of benzenesulfonic acid
The original commercial route was developed by Bayer and Monsanto in the early 1900s, based on discoveries by Wurtz and Kekulé. The method involves the reaction of a strong base with benzenesulfonate, proceeded by the reaction of hydroxide with sodium benzenesulfonate to give sodium phenoxide. Acidification of the latter gives phenol. The net conversion is:[Wittcoff, H.A., Reuben, B.G. Industrial Organic Chemicals in Perspective. Part One: Raw Materials and Manufacture. Wiley-Interscience, New York. 1980.]
Hydrolysis of chlorobenzene
Chlorobenzene can be hydrolyzed to phenol using a base (Dow process) or steam (Raschig–Hooker process):
These methods suffer from the cost of the chlorobenzene and the need to dispose of the chloride byproduct.
Coal pyrolysis
Phenol is also a recoverable byproduct of coal pyrolysis.[Franck, H.-G., Stadelhofer, J.W. Industrial Aromatic Chemistry. Springer-Verlag, New York. 1988. pp. 148-155.] In the Lummus process, the oxidation of toluene to benzoic acid is conducted separately.
Miscellaneous methods
Phenyldiazonium salts hydrolyze to phenol. The method is of no commercial interest since the precursor is expensive.
Salicylic acid decarboxylates to phenol.
Shipping
Phenol, which is produced and hence transported in large volumes, is shipped in a molten state below . The melting point is lowered and corrosive nature enhanced in the presence of small amounts of water. Typically, stainless steel containers and nitrogen-blanketing are required to prevent discoloration.
Exposure and potential toxicity
Exposure to phenol may occur in people living near , hazardous waste sites or factories manufacturing it.[
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Exposure to phenol through any form of ingestion or contact can produce systemic poisoning, with possible symptoms including transient brain stimulation, followed by coma and over minutes to hours following exposure.[ Other symptoms may include hemolytic anemia, profuse sweating, hypotension, arrhythmia, pulmonary edema, nausea, vomiting, and diarrhea.][ Chronic exposure to phenol or its vapor may cause Nephrotoxicity, skin lesions, or gastrointestinal disease.][
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If inhaled, ingested or by skin contact, phenol can enter the blood, possibly causing breathing problems, headaches, or sore eyes.[ from skin exposures can be decontaminated by washing with polyethylene glycol,]
Safety concerns have caused phenol to be banned from use in cosmetic products in the European Union
Besides its hydrophobic effects, another possible mechanism for the toxicity of phenol is the formation of phenoxyl free radical.
Uses
Chemicals
The major uses of phenol, consuming two thirds of its production, involve its conversion to precursors for plastics. Condensation with acetone gives bisphenol-A, a key precursor to and epoxy resins. Condensation of phenol, Alkylphenol, or Diphenol with formaldehyde gives , an example of which is Bakelite.
Phenol is also a versatile precursor to a large collection of drugs, most notably aspirin but also many and pharmaceutical drugs. Phenol is a component in liquid–liquid phenol–chloroform extraction technique used in molecular biology for obtaining from tissues or cell culture samples. Depending on the pH of the solution either DNA or RNA can be extracted.
Phenol is so inexpensive that it also attracts many small-scale uses. It is a component of industrial used in the aviation industry for the removal of epoxy, polyurethane and other chemically resistant coatings.
Topical anesthetic
Concentrated liquid phenol can be used topically as a local anesthetic for otology procedures, such as myringotomy and tympanotomy tube placement, as an alternative to general anesthesia or other local anesthetics.[ It is the active ingredient in some oral analgesia.][
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Concentrated phenol liquids are used for permanent treatment of ingrown toe and finger nails, a procedure known as a chemical matrixectomy. The procedure was first described by Otto Boll in 1945.
Nerve block
Phenol is used as a chemical denervation agent in analgesia treatment, such as for spasticity, arthritis, or cancer pain.[ Its effect on the nerve is to denature protein, diminish nerve fat and myelin content, and interrupt sensory transmission to the brain.][ If successful, pain relief may last for weeks to two years.][ Complications may include pain on injection, bleeding, or infection.][
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Clinical studies from 2023-25 reported that local injection of phenol (1.5–3 ml of 6% phenol in sterile water) at three to five sensory knee nerves was effective as a neurolysis treatment to relieve pain associated with chronic osteoarthritis.[
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History
Phenol was discovered in 1834 by Friedlieb Ferdinand Runge, who extracted it (in impure form) from coal tar.[F. F. Runge (1834) "Ueber einige Produkte der Steinkohlendestillation" (On some products of coal distillation), Annalen der Physik und Chemie, 31: 65-78. On page 69 of volume 31, Runge names phenol "Karbolsäure" (coal-oil-acid, carbolic acid). Runge characterizes phenol in: F. F. Runge (1834) "Ueber einige Produkte der Steinkohlendestillation," Annalen der Physik und Chemie, 31: 308-328.] Runge called phenol "Karbolsäure" (coal-oil-acid, carbolic acid). Coal tar remained the primary source until the development of the petrochemical industry. French chemist Auguste Laurent extracted phenol in its pure form, as a derivative of benzene, in 1841.[Auguste Laurent (1841) "Mémoire sur le phényle et ses dérivés" (Memoir on benzene and its derivatives), Annales de Chimie et de Physique, series 3, 3: 195-228. On page 198, Laurent names phenol "hydrate de phényle" and "l'acide phénique".] In 1836, Auguste Laurent coined the name "phène" for benzene;[Auguste Laurent (1836) "Sur la chlorophénise et les acides chlorophénisique et chlorophénèsique," Annales de Chemie et de Physique, vol. 63, pp. 27–45, see p. 44: Je donne le nom de phène au radical fondamental des acides précédens (φαινω, j'éclaire), puisque la benzine se trouve dans le gaz de l'éclairage. (I give the name of "phène" (φαινω, I illuminate) to the fundamental radical of the preceding acid, because benzene is found in illuminating gas.)] this is the root of the word "phenol" and "phenyl". In 1843, French chemist Charles Gerhardt coined the name "phénol".[Gerhardt, Charles (1843) "Recherches sur la salicine," Annales de Chimie et de Physique, series 3, 7: 215-229. Gerhardt coins the name "phénol" on page 221.]
The antiseptic properties of phenol were used by Sir Joseph Lister in his pioneering technique of antiseptic surgery. Lister decided that the wounds had to be thoroughly cleaned. He then covered the wounds with a piece of rag or lint
Meanwhile, in Carlisle, England, officials were experimenting with sewage treatment using carbolic acid to reduce the smell of sewage Cesspit. Having heard of these developments, and having previously experimented with other chemicals for antiseptic purposes without much success, Lister decided to try carbolic acid as a wound antiseptic. He had his first chance on August 12, 1865, when he received a patient: an eleven-year-old boy with a tibia bone fracture which pierced the skin of his lower leg. Ordinarily, amputation would be the only solution. However, Lister decided to try carbolic acid. After setting the bone and supporting the leg with splints, he soaked clean cotton towels in undiluted carbolic acid and applied them to the wound, covered with a layer of tin foil, leaving them for four days. When he checked the wound, Lister was pleasantly surprised to find no signs of infection, just redness near the edges of the wound from mild burning by the carbolic acid. Reapplying fresh bandages with diluted carbolic acid, the boy was able to walk home after about six weeks of treatment.
By 16 March 1867, when the first results of Lister's work were published in the Lancet, he had treated a total of eleven patients using his new antiseptic method. Of those, only one had died, and that was through a complication that was nothing to do with Lister's wound-dressing technique. Now, for the first time, patients with compound fractures were likely to leave the hospital with all their limbs intact
- — Richard Hollingham, Blood and Guts: A History of Surgery, p. 62
Before antiseptic operations were introduced at the hospital, there were sixteen deaths in thirty-five surgical cases. Almost one in every two patients died. After antiseptic surgery was introduced in the summer of 1865, there were only six deaths in forty cases. The mortality rate had dropped from almost 50 per cent to around 15 per cent. It was a remarkable achievement
- — Richard Hollingham, Blood and Guts: A History of Surgery, p. 63
Phenol was the main ingredient of the "carbolic smoke ball," an ineffective device marketed in London in the 19th century as protection against influenza and other ailments, and the subject of the famous law case Carlill v Carbolic Smoke Ball Company. In the tort law case of Roe v Minister of Health, phenol was used to sterilize anaesthetic packed in , in which it contaminated the anaesthetic through invisible micro-cracks and caused paraplegia to the plaintiffs.
Second World War
The toxic effect of phenol on the central nervous system causes sudden collapse and loss of consciousness in both humans and animals; a state of cramping precedes these symptoms because of the motor activity controlled by the central nervous system.[ Injections of phenol were used as a means of individual execution by Nazi Germany during the Second World War.][ The Experiments by Peter Tyson. NOVA] It was originally used by the Nazis in 1939 as part of the mass-murder of disabled people under Aktion T4.[ The Nazi Doctors , Chapter 14, Killing with Syringes: Phenol Injections. By Dr. Robert Jay Lifton] The Germans learned that extermination of smaller groups was more economical by injection of each victim with phenol. Phenol injections were given to thousands of people. Maximilian Kolbe was also murdered with a phenol injection after surviving two weeks of dehydration and starvation in Auschwitz when he volunteered to die in place of a stranger. Approximately one gram is sufficient to cause death.
Occurrences
Phenol is a normal metabolic product, excreted in quantities up to 40 mg/L in human urine.[The Beaver: Its Life and Impact. Dietland Muller-Schwarze, 2003, page 43 ( book at google books)]
Phenol is a measurable component in the aroma and taste of the distinctive Islay whisky,
Biodegradation
Cryptanaerobacter phenolicus is a bacterium species that produces benzoic acid from phenol via 4-hydroxybenzoate.
Naming
The word phenol is also used to refer to any compound that contains a six-membered aromatic ring, bonded directly to a hydroxyl group (-OH). Thus, phenols are a class of of which the phenol discussed in this article is the simplest member.
See also
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Bamberger rearrangement
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Claisen rearrangement
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Cresol
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Fries rearrangement
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Polyphenol
External links
